High frequency tube method and apparatus



May 3, 1966 A, STAPRANS ET AL 3,249,794 HIGH FREQUENCY TUBE METHOD AfiDAPPARATUS Original Filed Aug. 6, 1959 2 Sheets-Sheet 1 a VII/IllINVENTORS ARMAND STAPRANS BY GEORGE CARYOTAKIS ATTORNEY May 3, 1966 A.'STAPRANS ET AL 3,249,794 HIGH FREQUENCY TUBE METHOD AND APPARATUS 2Sheets-Sheet 2 Original Filed Aug. 6, '1959,

e w H 8.... 3213mm fizusmmu T s N 36+ 36+ 0 RB 3c W m M 0 w v QC 1 m Ao. w. NM m w M Tu AM 1 N A AG w a. 5 m A m UE .R D elm 2258mm 55:85 W26+ 86+ a woo 30 m w m N W Q- 7 b w cl W m w: on m2 on mm om a 398 w owm9 Tm; m2 N2 E L E United States Patent 3,249,794 HIGH FREQUENCY TUBEMETHOD AND APPARATUS Armand Staprans, Mountain View, and GeorgeCaryotakis, Los Altos, Calif, assignors to Varian Associates, Palo Alto,Calif., a corporation of California Original application Aug. 6, 1959,Ser. No. 832,402, now Patent No. 3,169,206., dated Feb. 9, 1965. Dividedand this application Aug. 29, 1962, Ser. No. 220,326

2 Claims. (Cl. SIS-5.43)

The present invention is a divisional application of copendingapplication Serial No. 832,402, filed August 6, 1959, now issued as US.Patent No. 3,169,206 'on February 9, 1965, and relates in general tohigh frequency tubes and more particularly to a novel high power,pulsed, UHF, broad band amplifier useful, for example, in applicationsas navigation and communication systems, as a driver for a linearaccelerator, and the like.

The tube of the present invention is a fixed tuned five cavity UHFklystron amplifier having a half power bandwidth of approximately 12% to14% with an RF efficiencyof 32%. This tube is approximately eleven feetin length and including only the evacuated structure Weighsapproximately 700 pounds. The tube will deliver at UHF frequency 8 to 10megawatts peak RF. power with an average RF. power of 30 kw.

' The principal object of the present invention is to provide animproved high frequency klystron amplifier tube apparatus which isrelatively simple of construction, relatively easy to handle, and whichwill have long operating life while delivering high peak and highaverage R.F. power.

One feature of the present invention is the provision of method andapparatus for broadbanding a fixed tuned multicavity klystron amplifierby preferentially lowering the Qs and stagger tuning the cavities in acertain manner whereby extremely broadband operation is obtained.

Other features and advantages of the present invention will becomeapparent upon a perusal of the following specification taken inconnection with the, accompanying drawings wherein,

FIG. 1 is a longitudinal view, partly in section, showing themulticavity klystron amplifier apparatus of the present invention,

FIG. 2 is a cross sectional view of the structure of FIG. 1 taken alongline 2-2 in the direction of the arrows,

FIG. 3 is a schematic diagram of a fixed tuned broadband klystronamplifier of the present invention depicting the preferential loading ofthe cavities,

FIG. 4 is a graph of the reciprocal of cavity loading versus frequencyfor the klystron amplifier of FIG. 3,

FIG. 5 is a graph of small signal gain versus frequency deviation of abroad band klystron amplifier tube apparatus of FIG. 3, and

FIG. 6 is a graph of large signal efficiency in percent versus frequencydeviation for the fixed tuned broad band klystron apparatus as depictedin FIG. 3. I

Referring now to FIG. 1 there is shown a longitudinal partly crosssectional view of a high frequency high power multicavity klystron tubeapparatus utilizing features of the present invention. Morespecifically, the tube comprises an elongated tubular metallic envelope1 having an electron gun assembly 2 at one end thereof for producing anddirecting the beam of electrons axially through the elongated vacuumenvelope 1 to an electron collector assembly 3 mounted at the other endof the elongated envelope 1. A plurality of cavity resonators 4 are pro-"ice V A beam focusing solenoid 5 envelopes the central portion of thetubes envelope for focusing the electron beam throughout the length ofthe tube apparatus. The free end portion of the cathode assembly 2 isinserted within an oil tank 6 and sealed therewithin via suitable matingflanges provided on the tube envelope 1 and the oil tank 6. The oil inthe oil tank serves to prevent electrical breakdown across the highvoltage anode to cathode insulator of the cathode assembly 2.

Electromagnetic energy which it is desired to have amplified by the tubeis fed to the first cavity 4 of the tube via coaxial line 7 and inputloop 8. This R.F. energy serves to velocity modulate the beam, suchvelocity modulation being transformed into current density modulation asthe beam travels down the length of the tube. The current densitymodulation is further enhanced by successive cavities 4. The currentdensity modulation serves to excite the last or output cavity 4. Thegreatly amplified RF. output energy is extracted from the output cavity4 via a suitable coupling iris 10 and hollow waveguide 9. The waveguide9 is wrapped around the collector assembly 3. The output R.F. energy isextracted from the waveguide 9 via a coaxial line 11 and thence fed viaa doorknob transition 12, having a cylindrical wave permeerably of thecoupled cavity type, serves to present a flat impedance at the outputgap to the beam over the desired frequency band. In particular, thebroadbanding method relates to the driver section and has to do with theproper selection of resonant frequencies and loaded Qs for theindividual driving cavities.

It has been found that, in general, to obtain broad band response thepoles of the drivingcavities in the complex frequency plane shouldpreferably lie in certain predetermined positions. Analysis ofmulticavity klystron amplifiers utilizing a complex frequency plane istaught in the following article: A Study of the Broadband FrequencyResponse of the Multicavity Klystron Amplifier, by K. H. Kreuchen, B. A.Auld and N. E. Dixon in Journal of Electronics, vol. 2, pp. .5295671957). If no zeros were present in the klystron response, then thesepoles should lie on a Tchebycheif ellipse. In practice, zeros will befound in the response, and therefore to counteract the effect of thezeros, the poles should be displaced from the ellipse in a certainmanner as follows.

It has been found that With multicavity klystrons the best broad bandresults are obtained when the Qs and frequency of the cavities arearranged, starting from the lowest frequency cavity, with the loaded Qdecreasing for each cavity successively higher in frequency until aboutthe center of the frequency band after which the loaded Q increasegradually, the highest frequency cavity having the highest Q. The exactpositions of the poles may be computed using the small-signalspace-charged-wave theory as taught by Kruchen, Auld and Dixon in theabove article.

A preferred distribution of loaded cavity Q versus frequency isindicated in FIG. 4, where letters A-E refer to individual drivingcavities. Qs of the cavities with regard to frequency as indicated inFIG. 4 will provide a substantial increase in the bandwidth of the tube.However, even greater enhancement Merely arranging the loadedin thebandwidth may be obtained if the cavities are related to the poles in acertain manner. More specifically, it has been found that the bandwidthof a multicavity klystron utilizing short drift lengths between cavitiesas of, for example, less than 60 of reduced plasma frequency phase asdefined by G. M. Branch et al. in General Electric Research LaboratoryReport No. 55RL1181A, February 1955, can be increased if the inputcavity 161 is chosen .at the lowest frequency. Furthermore, it has beenfound that the efliciency of the klystron is substantially improved ifthe one or two cavities immediately preceding the output cavity aretuned to the high frequency end of the band. Since one of theintermediary cavities preferably has a very low Q as of, for example 25,the problem of selecting the cavity tunings is uniquely determined for afour cavity driver. Loading a cavity to a very low Q for thisapplication is best accomplished by beam loading, i.e., making the gaplength in the order of 1 to 3 radians of electronic drift angle at theoperating frequency of the tube. If additional loading is desired, itmay be obtained by coupling an additional external load to the cavity asindicated in FIG. 3.

In particular, the second cavity 162 is preferably chosen as the lowestQ cavity because external loading is more easily accomplished near theinput cavity Where the RF. power is relatively low. The input cavity 161is preferably tuned to the low end of the band to increase thebandwidth. The last one or two cavities 163 and 164 are preferably tunedto the high frequency end of the band to enhance efiiciency. Generally,cavities with the lowest Q should be nearer to the input cavity. Thusfor a five cavity klystron amplifier having four driving cavities, thepreferred tuning arrangement is that the resonance frequency of thecavities should increase successively from the first cavity to the lastdriving cavity.

The five cavity fixed tuned tube model of the present invention havingloaded Qs as indicated in FIG. 3 and resonant frequencies of thecavities as indicated by the arrows in FIG. 5 yielded the small signalgain in db versus frequency deviation curve as plotted in FIG. 5 and thelarge signal efficiency in percent versus frequency deviation curve asplotted in FIG. 6. From these curves it is easily seen that a 12% to 14%bandwidth was obtained.

The physical construction of the five cavity fixed tuned tube model ofthe present invention utilized the tuners 21 only for initially tuningthe cavities to the frequencies as indicated in FIG. 5. External loadingwas provided for the second driver cavity as shown in FIG. 3.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a four-driver cavity klystron amplifier tube having thefour-driver cavities arranged in numerical sequence in the order inwhich they interact with the beam and including, a first cavity tuned toa resonant frequency atthe low end of the tube operating frequency band,second, third and fourth cavity resonators each successively tuned tohigher frequencies, the fourth cavity resonator tuned to the highestfrequency, and the second cavity having the lowest Q as loaded of thefour-driver cavities whereby broadband operation is facilitated.

2. The method of tuning a multicavity beam amplifier having a pluralityof successive driver cavities arranged along the beam path for broadbandresponse comprising the steps of, distributing the resonant frequenciesof the individual driver cavities over the desired operating band offrequencies, arranging the loaded Qs of the driver cavities, startingwith the lowert frequency cavity, with a decreasing Q with increasingfrequency, and continuing in this manner to an intermediate frequencycavity after which the loaded Qs of the cavities increase withincreasing frequency, and tuning the individual driver cavities whichare arranged along the beam from the beginning of the beam path towardthe end of the beam path in an ascending order of frequency wherebybroadband amplifier response is obtained.

References Cited by the Examiner UNITED STATES PATENTS 2,224,200 12/1940Schiememann 330154 2,591,910 4/1952 Barford 3l55.43 2,934,672 4/1960Pollack et al. 315-546 DAVID J. GALVIN, Primary Examiner.

1. IN A FOUR-DRIVER CAVITY KLYSTRON AMPLIFIER TUBE HAVING THEFOUR-DRIVER CAVITIES ARRANGED IN NUMERICAL SEQUENCE IN THE ORDER INWHICH THEY INTERACT WITH THE BEAM AND INCLUDING, A FIRST CAVITY TUNED TOA RESONANT FREQUENCY AT THE LOW END OF THE TUBE OPERATING FREQUENCY,BAND, SECOND, THIRD AND FOURTH CAVITY RESONATORS EACH SUCCESSIVELY TUNEDTO HIGHER FREQUENCIES, THE FOURTH CAVITY RESONATOR TUNED TO THE HIGHESTFREQUENCY, AND THE SECOND CAVITY HAVING THE LOWEST Q AS LOADED OF THEFOUR-DRIVER CAVITIES WHEREBY BROADBAND OPERATION IS FACILITATED.